scd:openpgp: Rename an internal variable.

* scd/app-openpgp.c (struct app_local_s): s/extcap_v3/is_v3/.
s/max_certlen_3/max_certlen.  Change users.

The extcap_v3 flag is set if the version is 3 or later and as such
does not only declare that the v3 extcap layout is used. Make this
clear by renaming.

Likewise for max_certlen_3.

• Signed-off-by: Werner Koch <wk@gnupg.org>